There is a growing need in today's marketplace for an economical, transparent packaging material which is an adequate barrier to moisture and oxygen and an excellent barrier to aroma and flavor. In addition, there is a need to provide these qualities in a film that is recyclable and thus environmentally friendly.
Oriented homopolymer polypropylene films provide many packaging advantages, including excellent barrier properties against greases, oils and water vapor. It is known that biaxially oriented polypropylene (BOPP) films can be modified in order to enhance light, oxygen, water vapor, flavor and aroma barrier properties. While many methods of modifying BOPP films are currently known, each has its disadvantage.
The current commercial practices for modifying BOPP film include metallization techniques and coating the film with various components. Typical coating components include polyvinylidiene chloride (PVDC), acrylic acid (AA), and polyvinyl alcohol (PVOH). Each one of these modification procedures requires an out-of-line process and the resulting product in each case is non-recyclable. Metallization of BOPP film by vacuum deposition of a very thin layer of aluminum provides excellent light, oxygen and water vapor barrier properties. Accordingly, such films are used extensively in the salty snack food and bakery markets. Unfortunately, these films do not provide the enhanced flavor and aroma barrier properties sought by many of today's packagers.
PVDC and AA-coated BOPP films have been in existence commercially for many years. Both films have high clarity. High barrier PVDC-coated films are employed when superior oxygen and aroma barrier properties are critical in a clear package. PVDC coatings also improve the water barrier vapor properties of a coated BOPP film. However, one problem with PVDC coatings is that the heat sealability of a PVDC-coated film is dependent on the level of functional comonomer in the latex composition. The higher the comonomer content in the PVDC latex, the better the sealability, but as sealability increases oxygen barrier properties diminish. As a result, very high barrier PVDC coatings are generally non-heat sealable.
Acrylic acid (AA) coatings provide high gloss, good sealability, adhesion enhancement when corona treated, and can be used as a flavor and aroma barrier. Unfortunately, producing AA coatings requires an out-of-line process, as discussed above, and are non-recyclable.
The most recent commercially available coated film technology involves coatings of polyvinylalcohol (PVOH). PVOH coatings are clear and provide excellent oxygen barrier properties, and can also be used as a flavor and aroma barrier to some extent. The oxygen barrier performance, however, is related to relative humidity and exposure to high moisture causes a significant deterioration in the oxygen barrier performance, probably because PVOH coatings are water soluble.
An additional emerging technology for a clear barrier BOPP film is the plasma-enhanced vapor deposition of a very thin layer of silica glass, e.g., SiO.sub.x wherein x can vary in the composition from 0.5 to 4.0. This technology is developmental for BOPP and is not commercially viable at this time due to problems with economics, flex crack resistance, and yellowness. SiO.sub.x coatings also are non-recyclable and thus not preferred for many applications.
A need therefore exists for a BOPP coating which provides excellent light, oxygen, water vapor, flavor and aroma barrier properties, yet is recyclable.
The sciences of water, light and oxygen permeation for BOPP films are generally well understood and have been quantified with commercially available testing equipment. The science of aroma barrier technology, on the other hand, involves more complex molecules, interactions and mechanisms that are not very well understood. Testing equipment is beginning to be commercialized which can quantify and detect permeants and flavors from single as well as multiple sources. The most reliable and basic testing equipment for detection of a permeant, however, is the human nose. The nose can detect odors at very low concentrations but cannot quantify levels in terms of parts per million (ppm) or parts per billion (ppb). However, the human nose can distinguish between slight and strong odors. Given a known permeant, a trained human nose can detect the presence or absence of that permeant with high certainty.
Heterogeneous polymer blends used as barriers to permeation of fluid, liquid and gaseous materials are taught in U.S. Pat. Nos. 4,410,482 and 4,444,817. These patents teach combining a polycondensation polymer with a polyolefin by using an alkylcarboxyl-substituted polyolefin as a compatabilizing material to form a heterogeneous melt. The compositions disclosed comprise 60 to 95% by weight polyolefin, 5 to 40% by weight condensation polymer incompatible with the polyolefin, and 0.25 to 12% by weight alkylcarboxyl polyolefin substituted with an unsaturated monomer having a carboxylic moiety grafted to a polyolefin. The patents teach processes of forming a continuous matrix phase wherein the condensation polymer, which is incompatible with the polyolefin, is present in the form of a discontinuous distribution of thin, substantially two-dimensional, parallel and overlapping layers, and the alkylcarboxyl-substituted polyolefin is present between the matrix and the layers and adheres the matrix and layers together. A relatively thick film results. These processes are known as laminar technology, and are further embellished in U.S. Pat. No. 4,416,942. The disclosures of each of these patents are herein incorporated in their entirety by reference.
While the foregoing permeation barriers find use in many applications, a need still exists for a barrier which is highly impermeable to aromas and flavors. In addition, a need also exists for a continuous homogeneous layer which provides excellent barrier properties in a commercially viable thin BOPP film.